The present invention relates to stopping the propagation of cracks in steel pipes, or pipelines used for and in gas conduits with a length of at least of at least 300 mm; a structure for impeding the propagation of such cracks should have an internal diameter equal to the internal diameter of the conduit. Moreover the particular impeding device should have a front face such that it can be butt welded in end-to-end configuration with a member of the conduit.
The conduction of natural gas or crude oil over large distances employs to a considerable extent longitudinally welded tubes or pipes. These pipes should withstand gas pressures of 60 bars and higher. The tubes, or pipes are made from thermo-mechanically rolled sheet or strip or skelp stock and should be amenable to a mechanical expansion for improving the shape and configuration.
In the foregoing the basic background for practicing the invention has been developed under particular consideration of the type of tubes, pipes and their mode of manufacture envisioned here to be improved. These conduits and pipe lines, depending to some extent on the environment in which they are used are to some extent endangered by formation of local cracks and fissures. Even though this may be a rare event a single event may have catastrophic results. This is particularly so as a crack once formed may exhibit the tendency of rapid propagation over long distances. Many proposals have been made and published in order to prevent crack growth and propagation.
European printed patent application No. 7,448,2 includes a comprehensive description of a variety of proposals that have been made here. Generally speaking the technical conditions under which crack propagation has to be prevented can be regarded as generally known. However, for a variety of reasons the various proposals are endowed with drawbacks and deficiencies relating for example to manufacturing techniques or to the effect that is hoped to be obtained but may not always be realized.
In accordance with a known proposal the various conduits should be constructed basically in a laminated fashion; the individual elements being threaded seam pipes. Such a pipe may as far as an individual section is concerned be indeed meet highest safety requirements provided each individual lamina fulfills completely the operating conditions. However, pipes and pipe lines of a thousand km length or more can from a realistic point of view simply not be constructed in this fashion.
Of greater advantage and particularly more realistic are conduits made of longitudinally welded tubes and pipes and wherein within the pipe line so called crack stoppers are included and inserted. Basically one can consider such a crack stopper to be a particularly configured, short pipe, tube or sleeve which replaces over a short section the regular pipe of the pipeline. Examples of such a crack stopper are e.g. shown in U.S. Pat. No. 3,096,105. Local reinforcement is an analogous way of proceeding and is disclosed in German Pat. No. 9,369,81. These proposals, however, are endowed with the drawback that they will be effective if and only if the wall thickness at the pipe at that particular location is strengthened and increased. The pipe, however, must offer in its interior a smooth flow space, without internal constriction, and as far as installation and surface contour is concerned the outside should be smooth too. Clearly any local increase in the tube or pipe wall thickness will not meet those requirements.